Various systems are employed on engines for detecting a misfire of a combustion event. If a cylinder repeatedly misfires, fuel is typically shut off to that cylinder. This prevents the passage of a large amounts of unburned fuel to an exhaust catalyst. This is done to prevent degradation of the catalyst's performance and useful life.
One type of system is coupled to an ignition system for detecting ignition related misfires. This scheme is deficient because it can only detect ignition related misfiring conditions which are a subset of the possible misfiring conditions and therefore lack the full function necessary to accurately determine misfire over a broad range of operating conditions.
Another scheme is to measure a temperature of the exhaust gas stream from an engine. Also, the content of carbon monoxide and hydrocarbons may be sensed to determine a misfire condition. Both of these schemes are plagued by slow response speeds of the sensory systems and the limited durability of the sensors in the hostile automotive environment.
Another scheme monitors average angular velocity of an engine's crankshaft. A signature analysis is performed on this average engine crankshaft velocity in an attempt to predict a misfire condition. Other schemes rely on measuring average engine crankshaft acceleration. Both of these schemes suffer from inaccuracy because they rely on multicombustion cycle averaging. This is problematic because these schemes are inaccurate and unreliable during transient operating conditions and other conditions with strong combustion variability. Combustion variability comes in many forms including crankshaft torsional effects, due to the resonant characteristics of the crankshaft, and effects of various engine accessories such as an alternator, an air conditioner compressor, a fan etc.
Additionally, the misfire component of the sensed signal varies considerably in magnitude and frequency over the full operating range of the engine. Since averaging schemes rely on predicting a change from a steady state condition they inherently loose accuracy under these transient operating conditions. Also, non-combustion related effects are substantial. These effects are typically attributable to variations in engine load torque induced by reciprocating inertia torque, and crankshaft torsional vibration.
Another scheme is to measure a pressure or flow fluctuation in an exhaust path of a combustion chamber, through the employment of a pressure transducer. Through signature analysis, an output of this pressure transducer is compared to a predetermined signal, for detecting a misfire condition.
Other systems have considered analysis of audio output from an engine. It relies on analyzing the engine firing performance by coupling an audio sensor to an output of an exhaust system for measuring a frequency spectrum of exhaust noises.
This scheme, and the former exhaust measurement scheme, also have many deficiencies. For instance, it is substantially dependent on the characteristics of the coupling medium, in this case the exhaust system. The exhaust system, includes an exhaust manifold, coupled to an exhaust tube, that is coupled to a catalytic converter, that is coupled to a muffler, that is coupled to an exhaust pipe. Because of this structure, this arrangement is susceptive to interference from non-engine performance related audio noise sources including engine and vehicle vibrations that are coupled into the exhaust system. A resonance of this coupling medium may add to the harmonic spectra provided by the engine. Also, because of its large volumetric size, the exhaust system acts like a low pass filter that reduces the available signal thus effecting the accuracy of the measurement. Additionally, the propagation time of audio output from the engine will change as the exhaust system heats up, or cools down. Further, accuracy under transient engine operating conditions will be impaired by the time lag associated with the length of the exhaust system. Because of this, engine synchronous tuning cannot be guaranteed. Also, the length that the individual cylinder audio output traverses varies with each cylinder because of the different exhaust runner lengths of the exhaust manifold. This will cause a variable delay from when the exhaust valve opens to when it is sensed. This variable length coupling from each cylinder to the sensory means may also shift the harmonic spectra provided by the engine. This is because of the pressure wave reflections that are caused by the different amount of time a pressure pulse will take to travel from an exhaust valve to the audio sensor in different cylinders. Further, an engine's exhaust system is tuned for optimal engine performance. By using this scheme, this tuning is more complex because of the additional concern of providing for an audio sensor in the tuning path. Also the audio sensor has durability limitations.
In summary, prior art misfire detection schemes are inaccurate, slow to respond to transient engine operating conditions, and incomplete in their ability to sense a broad scope of misfire conditions possible in an operating engine.
What is needed is an improved system for detecting misfire in internal combustion engines that is accurate, able to respond to transient engine operating conditions, able to sense a broad scope of misfire conditions possible in an operating engine, requires minimum calibration, and can be easily applied to different engine families.